During dialysis, it is often desirable to control and measure ultrafiltration, i.e., the loss of liquid by a patient resulting from passage of the liquid from the blood side of a semipermeable membrane to the dialysate side. Low flowrates, i.e., between 2 and 30 ml/min, are typically involved, and variations of the ultrafiltration flowrate are often caused by changes in blood pressure near the membrane caused by movement of the patient.
In measuring ultrafiltration, a flow of liquid equal in amount to the ultrafiltration is typically segregated from the much larger bulk dialysate flow, and one way of doing this is to stop, for a short period of time, the bulk flow of dialysate while maintaining the same dialysate and blood pressures so that the entire liquid flow leaving the dialyzer is owing to ultrafiltration.